Microstructure and Mechanical Properties of ASTM A743 CA6NM Steel Welded by FCAW Process

Abstract

CA6NM steel is widely used in the manufacture of hydraulic turbines metallic parts, due to its resistance to corrosion and cavitation damage, combined with good weldability and fatigue properties. However, welding of this type of steel is complex and to ensure a minimum residual stress after welding it is necessary perform a post welding heat treatment (PWHT) of the part. This study aims to analyze the effect of a PWHT on the microstructure and mechanical properties of CA6NM steel weld joint produced by the FCAW process and compare it with the characteristics of an as-welded joint. A martensitic microstructure has been present in both materials. However, the PWHT material has shown finely dispersed retained austenite, in an amount near 10 vol.%. Vickers microhardness values of all regions of PWHT welded joint present lower hardness values compared to those of the as-welded joint. Despite nearly identical toughness values of the weld metal from AW and PWHT samples, results of fracture analysis have shown distinct features in appearance of the fractures.

Vibratory Stress Relief and Vibratory Weld Conditioning of Flux cored arc welded CA6NM steel

“…This could be, at least in part, associated to a contribution from the base metal (absorbed energy ~ 97 J) in these results, assuming that the crack propagation path may include the base metal. The VWC and VSR samples present near identical absorbed impact energy values for both S and SS notch regions, located in the weld metal, in addition, very similar results were also found for the AW and PWHT samples (Silva et al, 2017). Figure 7.…”

“…Figure 5 presents the X-ray diffractograms from the weld metal of the VSR (Fig 5a) and VWC (Fig 5b) joint as well as those of PWHT (Fig 5c) and AW (Fig 5d) conditions for comparison. The results suggest a single-phase martensitic microstructure for all joints, except for the PWHT joint which presented near 10 % retained austenite (Silva et al, 2017) in addition to martensite. The reason for retained austenite in the weld metal of the PWHT joint should be related to different austenite initial composition when this material is reheated at 580 o C in the α + γ two-phase field.…”

“…Subsequent quenching and tempering were applied to produce the mechanical properties to the standard requirements. Table 1 shows the chemical composition of the CA6NM steel according to the ASTM A743/A743M standard as well as the chemical composition of the AWS E410NiMoT1-4/-1 (Silva et al, 2017) tubular wire (welding electrode),with 1.6 mm diameter, manufactured by Mckay Company attending to ASME/AWS SFA 5-22 specifications (AWS, 2012). Table 2 compares the mechanical properties of CA6NM steel used in this work to the requirements found in ASTM A743/A743M standard.…”

“…Non-Destructive Tests (NDT) by radiography were used to ensure the integrity of the welded joints. Joints for the condition as-welded, no vibration (AW code) and post weld heat treated (PWHT code) were produced similarly and the results published elsewhere (Silva et al, 2017).…”

“…The aim of this investigation was to analyze the effects of applying mechanical vibration during and after FCAW welding process of martensitic CA6NM stainless steel on its mechanical properties and to make a comparison with the conventional method in terms of microstructure and mechanical properties. Results for the as-welded (no vibration) as well as post weld heat treated (PWHT) joints were published previously (Silva et al, 2017).…”